Heat oil in a heavy 4-quart saucepan over med-high heat.
Add in onion, carrot, and celery; cook, stirring, until the vegetables begin to soften, 2-3 minutes.
Add in lentils, broth, and water; bring to a boil.
Partially cover the saucepan, decrease heat to med-low, and simmer until the lentils are soft and the soup is thick, about 45 minutes.
Transfer about half the soup to a food processor or blender or use an immersion blender, and pulse the machine a few times to lightly blend the soup; return soup to the saucepan, stir to combine with the unpureed soup, season with salt and pepper, and heat through before serving.